Recently, biochemistry has been markedly advanced, and analyses of substances that constitute organisms, such as proteins, and research of metabolic functions of substances, including drugs, have been actively pursued. There has been particularly marked progress in the study of molecular biology to clarify biological phenomena on the basis of the structure and function of biopolymers. In this research, analyses of substances that constitute organisms, such as DNA and proteins, are essential requirements.
Tracers are substances which are added to obtain knowledge about the behavior of elements or substances, and experiments that employ tracers are often conducted in the fields of medical science, pharmacology, etc. To trace the behavior of an element or a compound, it is common to employ a labelled compound that contains a radioisotope of the element concerned. In the medical and pharmaceutical fields, tritium (.sup.3 H), carbon (.sup.14 C), etc., which are organism-constituting elements, are the principal radioisotopes used.
In a tracer experiment, information about the behavior of a target element or compound is obtained by tracing the radioactivity of a radioisotope label. Tracer experiments are also widely conducted in the fields of biochemistry, medical science and pharmacology as stated above. It is a common practice in a tracer experiment to artificially add a radioisotope (e.g., a labelled compound) to an object of measurement. Since radioisotopes are also widely distributed in nature in fixed proportions, in the form of radioisotopes produced, for example, by nuclear reactions caused by cosmic rays, it is possible to estimate the age of an organism, by analyzing the proportion of .sup.14 C in the organism. Therefore, radioisotopes are also used in research for age determination.
Scintillation is the phenomenon whereby a fluorescent substance emits light when radiation is applied thereto. Scintillation counters are used to detect--rays, X-rays, .beta.-rays and neutron rays. Scintillation counters are arranged to convert radiation energy into light by means of a fluorescent substance, convert the light into electronic pulses by a photomultiplier tube and count the number of pulses. The above-described tracer experiments are generally conducted by the use of such scintillation counters.
Scintillation counters are capable of detecting and counting any kind of radiation, such as photon (--rays) and neutron rays, in addition to charged particles. Luminescent substances that are used in scintillation counters are called scintillators. The wavelengths of light emitted by scintillators are generally from 3000 to 600 Angstroms. Scintillators are required to have such characteristics that they have no absorption region in the above-described wavelength range, transmit fluorescence life have a short fluorescence lifetime (which sets a limit on the decomposition time).
Scintillators may be divided into the following classes according to the phase in which they are used: solid scintillators, liquid scintillators and gas scintillators. A great variety of such scintillators have heretofore been proposed and used. For example, the invention of thallium-activated sodium iodide [NaI(Tl)], which is crystalline, was the beginning of the gamma spectroscopy. Additional examples of scintillators are thallium-activated cesium iodide [CsI(Tl)], sodium-activated cesium iodide [CsI(Na)] and europium-activated lithium iodide [LiI(Eu)].
Silver-activated zinc sulfide [ZnS(Ag)], europium-activated calcium fluoride [CaF.sub.2 (Eu)], bismuth germanate (BGO, Bi.sub.4 Ge.sub.3 O.sub.12) and cesium fluoride (CsF) are also known. In addition, a glass scintillator which is made of quartz is known. Gas scintillators that employ rare gases, such as xenon, helium, etc., are considered to be high-speed scintillators, but they are usually used only for special purposes.
Liquid scintillators are often employed at the present time because they are liquids and therefore easy to handle, and they also make it possible to obtain a geometry which is advantageous to the measurement of .sup.3 H and .sup.14 C. The main components of liquid scintillators are an organic solvent and a dissolved fluorescent substance, but some liquid scintillators also contain a surfactant for emulsifying the organic solvent or some other additives.
The characteristics of liquid scintillators are determined by the kind of solvent, dissolved substance, surfactant, etc. selected and the amount of each material used. If, for example, xylene or toluene is employed as the solvent in a liquid scintillator, the resulting scintillator may be called a xylene-based or toluene-based liquid scintillator. A liquid scintillator which contains a surfactant may also be called an emulsifying scintillator.
As will be understood from the foregoing description, scintillators may also be considered to be substances functioning as energy transducers that convert radiation energy into fluorescent energy. In preparation of a specimen, it suffices to incorporate a radioactive substance into a solution which is in the form of a scintillation cocktail.
The solid, liquid and gas scintillators described above are all used in such a state that they are irradiated with radiation emitted from objects of measurement. Many liquid scintillators are generally used in the form of a mixture with an object of measurement which is contained in a glass container.